This invention relates in general to gas sensing devices and in particular to a new and useful device for sensing oxygen content in a gas mixture as a result of the variations of paramagnetism producing induction variations in a bridge circuit.
Oxygen is used in many gas reactions as reactant and in respiratory apparatus as an important gas component. For a long time, there is a need for continusously determining the O.sub.2 concentration in the various streams and reaction stages. Since the other gas components may strongly differ in their nature and concentration, an analyzer is wanted which is as selective as possible.
An oxygen property of interest in this regard is its paramagnetic susceptibility which in many respects by far exceeds that of most of the technological gases. Therefore, O.sub.2 measuring methods based on magnetism are predominant. The three most employed are:
(a) the thermomagnetic method,
(b) the method based on the principle of so called "Remling's" magnetic torsion balance, and
(c) the rise equilibrium method.
Apparatus according to (a) have proved as rugged and reliable measuring devices. However, they have the disadvantage that accompanying gases, especially at low concentrations, invalidate the measurement, depending on the nature and concentration of the gas.
"Remling's" magnetic torsion balance according to (b), comprises two thin-walled balls which are connected to each other by thin rods and suspended from a torsion thread in a field produced by two magnet pole couples. The magnet poles and the balls are enclosed in a measuring chamber. The angle of deviation is a measure of the O.sub.2 content in the measured gas. The difficulties with a torsion balance method arise from the extraordinarily small torsional moment to be measured and the sensitivity of the apparatus to external shocks and vibrations.
In the rise equilibrium method according to (c), a gas stream to be measured and an O.sub.2 -free reference gas stream are introduced into a magnetic field, and the differential pressure between the inlet connections and the two streams is measured. It is disadvantageous that the differential pressure to be measured is very small. A particular advantage, however, is the quick response to rapid concentration variations in the measured gas.
Still another method is known, utilizing the paramagnetic property of oxygen. In this method, the gas mixture is introduced in the interior of a radio-frequency coil and serves virtually as a coil core. Consequently, the self-inductance of the coil varies in accordance with the proportion of paramagnetic components. This variation of the self-inductance of the coil brings a radio-frequency circuit out of tune, and the amount of detuning is a measure of the proportion of paramagnetic substances. What is disadvantageous in this method is the small amount of the effect to be utilized, and the measuring difficulties resulting therefrom (German Pat. No. 871539).